This invention is concerned with an apparatus for simultaneously measuring the outer diameter of sets of profiled wheels which have been produced by a turning operation.
It is known from U.S. Pats. Nos. 3,307,265, 3,561,121 and 3,561,120 to carry out length measurements by means of metering wheels. In these prior-art devices, the metering wheels roll on precisely planar metering surfaces of a lathe or similar bed. These surfaces must be highly accurate if metering inaccuracies are to be avoided. Despite this, the cooperation between the surfaces and the metering wheels results in measuring inaccuracies which, after a to-and-fro movement of the respective metering wheel, make it difficult to place the zero-setting mark of the metering wheel in registry with the zero-setting mark of the surface on which the wheel is to roll. The overcome this problem, the prior-art devices form their metering wheels with convexly shaped circumferential surfaces.
These metering wheels may be pivoted about a pivot axis that extends vertically with respect to the metering surface, about a pivot axis that extends parallel to the elongation of the metering surface, or about both such axes. This permits the necessary adjustment of the metering devices to be made. The circumferential surfaces of the metering wheels are formed with transverse grooves which cooperate with the metering surface to more precisely guide the wheels thereon. The wheels drive a rotating signal-generating device which yields signals that are supplied to a counter from which the length of the distance being measured can be directly read off.
It has also been proposed to use such arrangements for measuring the diameter of an object. In all instances of devices using metering wheels, it has always been exclusively proposed to measure a single diameter or a single length in a certain measuring system (for purposes of this disclosure it will be assumed that the metric system is involved). This means that the metering device must also operate in the same measuring system, which means that metering-wheel diameter, dial divisions, etc. all have a fixed relationship to the measuring system in question, i.e. here the metric system. If the metering wheel becomes worn, as it inevitably does, this fixed relationship is lost and incorrect measurements will result.
A result of this is that in the prior-art devices which use metering wheels, the metering wheels must be replaced with new ones when they become worn to even a slight extent. This is expensive and in many instances will also require undesirable machine down-time. Of course, a certain amount of wear may be considered acceptable, but this requires that a concomitant degree of measuring error must also be tolerated.
This presents a substantial problem if such a device is to be used to measure the diameter of a set of wheels, especially rail-vehicle wheels, since the rather rough surfaces to be measured cause a very significant amount of wear for the metering wheels. This is expecially true because the metering wheels must be pressed with a substantial force against the wheel surfaces being measured, to avoid "jumping" during the measuring operation and the concomitant measuring error resulting therefrom.
When a set of such wheels is to be measured, this is done to obtain accurate measurements of the diameters of both of the wheels. If this is done with metering wheels, they do not encounter either a planar surface of a cylindrical surface, as in the prior-art applications. Instead, the surfaces to be measured on such sets of wheels (e.g. rail-vehicle wheels) are rounded and may have cone angle which will differ from set to set if the same metering-wheel device is to be used to measure the diameters of wheel sets having different profiles. The surface quality of the wheels is produced in accordance with economic considerations, and does not take into account measuring accuracy. The surfaces to be measured are formed by a turning operation, which means that the cutting tool employed in this operation has formed these surfaces with spiral circumferentially extending grooves whose width and depth may vary from wheel set to wheel set, and even from wheel to wheel of one and the same set. The reason for the last-mentioned differences is that the turning machine, on which both wheels of a set are produced simultaneously, may have a differential feed rate for the tool associated with each wheel, that the cutting edges on the two tools may be of different dimensions, and that the degree of wear of these cutting edges may be different for the two tools.
It is known from "Industrieanzeiger", 1969, Nr. 79, pages 1925 ff, in an article entitled "Reibraddurchmesserverfahren", to use metering wheels to measure the outer diameter of rail-vehicle wheels. According to that disclosure, two metering wheels are used having axes which extend vertical with respect to the plane that encompasses the wheel diameter to be measured. The diameters of the metering wheels are produced very precisely. The metering wheels are coupled with a rotary signal generator which, in response to rotation of the metering wheels, generates for each rotation which it performs a number of signals that is so selected that the resolution capability of the measuring device in combination with the number of turns performed by the workpiece yields a decade number.
The known metering wheels of the prior art never have the width of their cylindrical circumference related to the measuring results. This means that this width is determined solely by manufacturing considerations. Similarly, the shape or the configuration of the cylindrical part of the metering wheel circumference to the axial endfaces has heretofore been completely disregarded with respect to the measuring results being obtained. It is, however, desired in the prior art that the axial center plane of the cylindrical part of the metering wheel coincide with the plane encompassing the diameter to be measured on the wheels of a wheel set.
However, those edges of the cylindrical part of the metering wheels which e.g. face toward the flange of wheel being measured, are located not in the plane of the diameter being measured, but in a different plane. This means that the diameter being measured by the metering wheel is not the diameter in the desired measuring plane, but a different diameter, so that the measured result will deviate from the result that should have been obtained and the correct diameter been measured. This deviation might be acceptable if shape-, positional- and measuring deviations were identical for both metering wheels of the measuring device. This, however, is not the case since the metering wheel widths and other dimensional factors are not controlled to be uniform, so that the planes in which the workpiece wheels are measured not only deviate from the plane of the desired diameter measurement, but deviate differently therefrom in respect of each of the metering wheels. It should be remembered that each metering wheel rolls with its circumference on the turned surface of the associated workpiece wheel, and more particularly, that it rolls on the crests of the circumferentially extending spiral grooves formed in this surface during the manufacture of the workpiece wheel. However, only those grooves will yield the accurate diameter whose crests, seen with reference to the circumference, are equally bisected into two parts by the measuring plane, i.e. the plane in which the diameter measurement is being taken. These grooves are very shallow and the rounded edge at that axial end of the metering wheel which faces towards the workpiece-wheel flange can enter into the groove and abut its bottom, whereas accurate measurement would require that the cylindrical part of the metering wheel circumference straddles the groove and rolls on the crests located at both axial sides of the groove.
These problems are aggravated by the fact that where a metering wheel has a cylindrical measuring-surface part whose width is narrower than the axial spacing between the crests bounding the opposite sides of the grooves in the workpiece wheel, the metering wheel will dip into the grooves and thus yield an inaccurate measurement, since the measurement is not taken, as desired, over the crests but in the grooves bounded by them. Moreover, it is never certain that the axial spacing of the crests bounding the respective grooves is identical on the two wheels of a set being measured, so that the metering wheel measuring one workpiece wheel may dip into the grooves thereof, whereas the metering wheel associated with the other workpiece wheel may measure the diameter over the crests thereof.
The problems encountered with the question of measuring accuracy are largely based upon the fact that the measuring accuracy is an important factor in the level of the peak-to-valley height required for the workpiece wheel surfaces.
The state-owned West German railroad, for example, requires a peak-to-valley height (Rt) smaller than 60 micrometers, at a permissible diameter difference of .DELTA. D = 0.3 mm. However, as the foregoing discussion has indicated, the measuring inaccuracies of the known metering-wheel devices are such as to fall within the impermissible range, so that mismeasurements cause unnecessary loss of expensive workpiece-wheel material.
The diameter-measuring problems could be reduced if the peak-to-valley height (Rt) of the turned surfaces on the workpiece wheels were to be improved. However, attempts to do so encounter economic considerations having to do with the manufacture of the workpiece wheels and precluding any significant improvements in the Rt factor. Therefore, this is not a viable solution to the problem.